This book first addresses a comprehensive review of sheep mastitis. The authors covers up-to-date reports on aetiology and epidemiology, important features of the disease, such as pathogenesis, clinical signs and diagnosis, and the current knowledge on treatment and prophylactic strategies for mastitis control. The following study examines listeriosis, a life threathening disease. Infections are caused by Listeria monocytogenes, which enters by inhalation, orally and by ocular mucosa. Normally, disease will not be produced if the host is immunocompetent, but when the immune system is depressed. Aiming to evaluate lesions caused by listeriosis in sheep from 2013 to 2017, all cases sent to the Research Center of Advanced Studies in Animal Health were analyzed. The concluding investigation aims to identify diseases transmitted by Melophagus ovinus, since it serves as vector of pathogenic diseases such as Trypanosoma melophagium, Anaplasma ovis, Blue tongue virus, Bartonella schoenbuchensis, Bartonella chomelii, Bartonella melophagi, and Bartonella spp all over the world.